Triple negative/basal-like breast cancer (BLBC) is a hard to treat, and highly malignant BC subtype that is particularly prevalent in younger African American patients. In spite of the identification of the epidermal growth factor receptor (EGFR) as a major oncogene in these tumors, the etiology of BLBC and the mechanisms underlying the frequent relapse and metastasis remain poorly understood. At the molecular level, the onset and progression of BLBC may require an interplay of tumor suppressors and/or oncogenes that together promote tumor malignancy. The tumor suppressor annexin A6 (AnxA6), has been shown to be down-regulated in highly malignant forms of BC, gastric cancer and melanomas. Although low AnxA6 expression is associated with poor overall survival of BLBC, it is not clear whether loss of AnxA6 per se, is sufficient to generate or maintain the malignancy of these tumors. Our preliminary data reveal that loss of AnxA6 in AnxA6-high and mutant p53 expressing BC cells is not only associated with early onset and rapid growth into large debilitating xenograft tumors in nude mice, but also rapid internalization and degradation of activated EGFR and hence, decreased cell motility. Meanwhile, up-regulation of AnxA6 in the AnxA6-low and p53-null HCC1806 cells promoted EGF- stimulated cell motility but attenuated the growth of xenograft tumors in nude mice. We also show that loss of AnxA6 may be triggered by hypoxia; and that the rapid growth of AnxA6-low tumor cells is partly driven by increased angiogenesis and/or activation of Ras by the Ca2+-activated RasGRF2. Interestingly, potent inhibition of Ca2+ influx via Ni2+-sensitive Ca2+ channels is associated with up-regulation of AnxA6 and down-regulation of RasGRF2 in the surviving BC cells. Based on these findings, we hypothesize that loss of AnxA6 and p53 deletion or inactivation drives the onset and aggressive growth of basal-like breast tumors; and that up-regulation of AnxA6 and decreased proliferation of a subset of BLBC cells following potent inhibition of Ca2+ influx or cytotoxic treatments underlies the frequent relapse of these tumors. We propose the following 3 specific aims: 1) to determine the significance of AnxA6 deficiency in basal-like breast tumor onset and progression in vivo using an Anxa6/Trp53 double knock out mouse model, and to elucidate strategies to attenuate the progression of the ensuing tumors; 2) to determine whether AnxA6 stabilization of membrane lipid rafts in BLBC cell lines underlies the frequent relapse and resistance to EGFR-targeted therapies or cytotoxic treatments; and 3) to determine whether the expression status of AnxA6, RasGRF2 and activated EGFR can predict the growth potentials and/or responsiveness of BLBC to EGFR-targeted therapies using a preclinical mouse model with tetracycline-inducible AnxA6 expression in AnxA6-low basal-like BC cell lines.